While electronic countermeasure systems had been in use for centuries in the Inner Sphere, towards the end of the Age of War these systems had become ubiquitous, making it much harder for units to find and target each other until it was too late. In an effort to combat this fog of war, the Star League developed the Beagle Active Probe to cut through the haze of electronic jamming and locate units quicker and sooner than before. However the League soon became concerned that it's Member States would acquire the Beagle and use it against them, leading to the development of the Guardian ECM Suite.[1]

The Guardian ECM Suite proved to be a powerful electronic warfare device, jamming the Beagle and other advanced electronic systems, but maintaining the system proved too hard for the Great Houses after the fall of the Star League. The system went extinct in the Inner Sphere in 2845, until it was eventually recovered by the Capellan Confederation in 3045.[2]

Before the Helm Memory Core was decoded, the Capellan Confederation tried to re-create the Star League-era Guardian ECM suites. The resulting electronics package was known only as ECM Equipment when it appeared on the Raven in 3025.[3] Given the technology level of the Succession Wars era, it isn't surprising that this equipment was heavier and bulkier than the systems that inspired it. Interestingly, it could operate in either an offensive or defensive capability: It could seek targets like a Beagle, or hide the unit mounting it like a Guardian. As it weighed as much as a standard PPC, took up as much room as an Autocannon/5, and had only half the range of the Guardian, production halted when the Confederation began production of their own Guardian systems in 3045.[2]

The information after this notice comes from apocryphal sources; the canonicity of such information is uncertain. Please view the reference page for information regarding their canonicity.

In MechWarrior IV: Mercenaries, the ECM is a selectable component for certain 'Mechs. It increases missile lock time for enemies and reduces the range at which the 'Mech can be detected.

In MechCommander 2, the ECM is also a component for certain 'Mechs. It makes the 'Mech in question undetectable by sensors. It appears to have no effect on missiles, due to the way in which the game handles weapons fire.